Printing is one of the most popular ways of conveying information to members of the general public. Digital printing using dot matrix printers allows rapid printing of text and graphics stored on computing devices such as personal computers. These printing methods allow rapid conversion of ideas and concepts to printed product at an economic price without time consuming and specialised production of intermediate printing plates such as lithographic plates. The development of digital printing methods has made printing an economic reality for the average person even in the home environment.
Conventional methods of dot matrix printing often involve the use of a printing head, e.g. an ink jet printing head, with a plurality of marking elements, e.g. ink jet nozzles. The marking elements transfer a marking material, e.g. ink or resin, from the printing head to a printing medium, e.g. paper or plastic. The printing may be monochrome, e.g. black, or multi-coloured, e.g. full colour printing using a CMY (cyan, magenta, yellow, black=a process black made up of a combination of C, M, Y), a CMYK (cyan, magenta, yellow, black), or a specialised colour scheme, (e.g. CMYK plus one or more additional spot or specialised colours). To print a printing medium such as paper or plastic, the marking elements are used or “fired” in a specific order while the printing medium is moved relative to the printing head. Each time a marking element is fired, marking material, e.g. ink, is transferred to the printing medium by a method depending on the printing technology used. Typically, in one form of printer, the head will be moved relative to the printing medium to produce a so-called raster line which extends in a first direction, e.g. across a page. The first direction is sometimes called the “fast scan” direction. A raster line comprises a series of dots delivered onto the printing medium by the marking elements of the printing head. The printing medium is moved, usually intermittently, in a second direction perpendicular to the first direction. The second direction is often called the slow scan direction.
The combination of printing raster lines and moving the printing medium relative to the printing head results in a series of parallel raster lines which are usually closely spaced. Seen from a distance, the human eye perceives a complete image and does not resolve the image into individual dots provided these dots are close enough together. Closely spaced dots of different colours are not distinguishable individually but give the impression of colours determined by the amount or intensity of the three colours cyan, magenta and yellow which have been applied.
In order to improve the veracity of printing, e.g. of a straight line, it is preferred if the distance between dots of the dot matrix is small, that is the printing has a high resolution. Although it cannot be said that high resolution always means good printing, it is true that a minimum resolution is necessary for high quality printing. A small dot spacing in the slow scan direction means a small distance between marker elements on the head, whereas regularly spaced dots at a small distance in the fast scan direction places constraints on the quality of the drives used to move the printing head relative to the printing medium in the fast scan direction.
Generally, there is a mechanism for positioning a marker element in a proper location over the printing medium before it is fired. Usually, such a drive mechanism is controlled by a microprocessor, a programmable digital device such as a PAL, a PLA, a FPGA or similar although the skilled person will appreciate that anything controlled by software can also be controlled by dedicated hardware and that software is only one implementation strategy.
One general problem of dot matrix printing is the formation of artefacts caused by the digital nature of the image representation and the use of equally spaced dots. Certain artefacts such as Moiré patterns may be generated due to the fact that the printing attempts to portray a continuous image by a matrix or pattern of (almost) equally spaced dots. One source of artefacts can be errors in the placing of dots caused by a variety of manufacturing defects such as the location of the marker elements in the head or systematic errors in the movement of the printing head relative to the printing medium. In particular, if one marking element is misplaced or its firing direction deviates from the intended direction, the resulting printing will show a defect which can run throughout the print. A variation in drop velocity will also cause artefacts when the printing head is moving, as time of flight of the drop will vary with variation in the velocity. Similarly, a systematic error in the drive system for moving the printing medium may result in defects that may be visible. For example, slip between the drive for the printing medium and the printing medium itself will introduce errors.
Especially in large size inkjet printers and industrial inkjet printing machines, the receiving medium transport system has to be very accurate and reliable in transport distance to avoid banding problems.
These systems usually must be capable to handle different sizes and thickness of receiving media.
Another problem is that the printing speed and transport speed is much higher than those of office or home inkjet printers.
These industrial printers often use a web-based material as printing stock. The web based material has to be fed very correctly as small deviations would lead to skew feeding of the web which could lead. to malfunctioning of the printer. Small feeding deviations in sheet-fed material do not pose such a problem as each sheet is independently taken from the paper bin, unless sheet-fed material is pre-printed and is to be accurately aligned in the printer to register the image to be printed to the already pre-printed image.
A problem also encountered is that printing on large size rigid media poses specific problems in respect to positioning and transporting of the media.
Rigid media normally have a greater weight than paper and have greater inertia than light materials which poses greater needs on the media transport system.
Due to the rigidity it is also possible that the material can not be straightened out easily and due to unevenness of the material surface the throw distance may vary and certain printing defects can occur.                Certain rigid materials exhibit a certain porosity so that they can not be easily transported by a transport system using vacuum forces to hold a medium. This problem is very apparent when one wants to print on mesh material, rigid or flexible.        
Another aspect in industrial printers is that the shuttle containing the printheads is usually relatively heavy in comparison to home or office printers. Due to the higher shuttle speed, the drops follow a sloping path from the printhead to the receiver. Even the slightest deviation in throw distance between the head and the receiver will result in deviations in positioning the ink drops. The throw distance has to be kept constant over the full width of the shuttle and over the full length of the shuttle movement.
It has been shown that transport rollers do not provide a solution to the problems described. Another drawback is that when using large size receiving media rollers are needed in the middle of the receiving medium and that these rollers come into contact with the fresh printed surface.
In WO 01/56 804 a conveyance apparatus is provided for stepwise conveying of materials which can be used in an inkjet printer. The apparatus uses fixed and moving elements for holding the working portion of the material, being the portion of the conveyed material on which the tool, in this case the inkjet printhead, is working on. The apparatus of WO 01/56 804 has however certain drawbacks.                Support of the working portion of the receiving medium is always divided over several elements of which some completely static and some are movable for transporting the receiving medium. The support structure is formed by the movable and fixed elements Therefor it can not be assured that the material is supported over the whole width at the same height and with the same force.        
Especially when printing thin, flexible media this would lead to problems.                As the moving elements are in contact with the receiving medium at the printing location no movement of these elements is tolerated during printing as longitudinal forces would be exerted upon the receiving medium at the printing location. This inevitably leads to a slower feeding speed.        The apparatus is riot able to transport materials having high porosity and mesh-like materials which are not laminated to a liner fabric.        the vacuum transport elements support only about 50% of the width of the material which gives possibly not enough force to move the heavier or porous materials.        
It is clear that there is still a need for improvement of these transport systems.
It is the aim of the invention to provide a receiving media transport system that can handle all types and sizes of receiving media having a very exact positioning capability.